Composite material for passive radiative cooling

ABSTRACT

A composite material for passive radiative cooling including a base layer, and at least one emissive layer located adjacent to a surface of the base layer, wherein the at least one emissive layer is affixed to the surface of the base layer via a binding agent.

RELATED APPLICATION DATA

This application is a continuation of U.S. application Ser. No.15/172,304, filed Jun. 3, 2016, which claims the priority benefit ofU.S. Application No. 62/170,369, filed Jun. 3, 2015, which are herebyincorporated in their entirety herein by reference.

TECHNICAL FIELD

The present disclosure relates to passive coolers which are capable ofcooling a supported article by radiation to the surrounding environment;and more particularly to a composite material for passive radiativecooling.

BACKGROUND

Radiative cooling refers to the process whereby a body will emit asradiation heat energy absorbed through normal convection and conductionprocesses. Generally, there is a low absorption “atmospheric window” inthe region of 8-13 μm where the atmosphere is relatively transparent. Asimilar window exists for some wavelengths within the 1-5 μm band.Radiation from the Earth's surface within these wavelengths is likely topass through these atmospheric windows to space rather than be absorbedby the atmosphere and returned to the Earth's surface.

For the wavelengths having high atmospheric absorption there will besignificant amounts of radiation in the atmosphere as that radiation isabsorbed and re-emitted back to Earth. Conversely, for the wavelengthscorresponding to these atmospheric windows there will be littleradiation in the atmosphere as the majority of radiation emitted by theEarth at these wavelengths is allowed to pass through the atmosphere tospace.

A “selective surface” is one that exploits the atmospheric window bypreferentially emitting thermal energy at wavelengths corresponding tothese atmospheric windows where there is reduced incident radiationwhich may be absorbed by the surface, that allows rapid transfer of thatradiation to space, and by that is non-absorptive of radiation outsidethese wavelengths.

Radiative cooling can include nighttime cooling; however, such coolingoften has a relatively limited practical relevance. For instance,nighttime radiative cooling is often of limited value because nighttimehas lower ambient temperatures than daytime, and therefore, there isless of a need for cooling. There is therefore a need for improvementsin composite materials to passively cool terrestrial structures such asbuildings, homes, electronics and other objects in both the daytime andthe nighttime.

SUMMARY

In some embodiments, a composite material for passive radiative coolingis provided. In some embodiments, the composite material comprises abase layer and at least one thermally-emissive layer located adjacent toa surface of the base layer. In some embodiments, the at least oneemissive layer is affixed to the surface of the base layer via a bindingagent.

In some embodiments, a composite material for passive radiative coolingis provided. In some embodiments, the composite material comprises abase layer and at least one thermally-emissive layer located adjacent toa surface of the base layer. In some embodiments, the surface of thebase layer comprises a reflective substrate comprising an adhesivelayer. In some embodiments, the at least one emissive layer is affixedto the base layer via the adhesive layer of the base layer.

Other embodiments are also disclosed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a schematic cross-sectional view of a compositematerial for passive radiative cooling according to one embodiment ofthe present disclosure;

FIG. 2 illustrates a cross-sectional view of a microparticle accordingto one embodiment of the present disclosure;

FIG. 3 illustrates a graph of a cooling potential of composite materialsagainst ambient daytime temperatures; and

FIG. 4 illustrates a graph of a cooling potential of composite materialsagainst ambient nighttime temperatures.

DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

To enhance the emissivity in the 8-13 μm wavelength range or in thewavelength range supported by a blackbody with temperatures in the rangeof 250−350° K, a composite material, generally indicated at 10 isapplied to the surface of an object. This leads to the preferentialemission of light in the 8-13 μm range or in the wavelength rangesupported by a blackbody with temperatures in the range of 250-350° K.The preferential emission of light is embodied in the emissivityspectrum.

In some embodiments, the composite material 10, as shown in FIG. 1,includes a base layer 12 composed of a reflective substrate composed ofat least one of aluminum, silver, glass, polyurethane, nylon, andpolyethylene fibers. In some embodiments, the reflective substratecomprises paint. In some embodiments, the paint comprises white paint.In some embodiments, the reflective substrate comprises glue. The baselayer 12 may be placed in thermal contact with an object to be cooled.

Immediately above the base layer 12 is at least one emissive layer 14 insome embodiments. The at least one emissive layer 14 may be arranged ina hexagonal monolayer, square monolayer, irregular monolayer, orirregular combination of between one and ten layers; exposed to sunlightand also to the atmosphere and paths for radiating thermal energy. In anembodiment, the at least one emissive layer 14 is composed of aplurality of microparticles 16. In one embodiment, each of the pluralityof microparticles 16 may be formed in a geometric shape, and composed ofa silica material. For example, the at least one emissive layer 14 mayinclude a plurality of microspheres. The plurality of microparticles 16may also be formed in square, cylindrical, or an irregular geometricshape to name a few non-limiting examples.

In an embodiment, with reference to FIG. 2, each of the plurality ofmicroparticles 16 includes a characteristic dimension 18. In oneembodiment, the characteristic dimension 18 is between about 5 to about50 microns. In another embodiment, the characteristic dimension 18 isless than 30 microns. In still another embodiment, the characteristicdimension 18 is between about 10 and about 20 microns.

In an embodiment, with reference to FIGS. 1 and 2, the at least oneemissive layer 14 is bonded to the base later 12 via a binding agent 20.The binding agent 20 may be composed of a transparent, polymer material.In some embodiments, the binding agent 20 may be uniformly applied tothe at least one emissive layer 14 or applied to each of the pluralityof microparticles 16, as shown in FIG. 2, via an electromagnetic brush(EMB) process or other suitable process. In some embodiments, the EMBprocess of the present disclosure utilizes a rotating wire brush inconjunction with static electromagnetic fields to deposit the pluralityof microparticles 16 to the base layer 12. In some embodiments, the EMBprocess requires the use of the binding agent 20 coating each of theplurality of microparticles 16 to achieve adhesion under subsequentheating. When applied to each of the plurality of microparticles 16 viaan electromagnetic brush (EMB) process, the binding agent 20 may have acharacteristic thickness 22 between about 1 and about 50 microns in oneembodiment. The binding agent 20 is melted after application of theplurality of microparticles 16 to the emissive layer 14 to form thebinding agent 20 layer illustrated in FIG. 1.

In some embodiments, a dry dusting process, rather than the EMB process,is utilized by dry dusting the plurality of microparticles 16 over thebase layer 12. The dry dusting process achieves a rough approximation ofthe uniform thin layer using a standard powder duster or squeeze bottlefilled with the plurality of microparticles 16. In some embodiments, thedry dusting process is used with the binding agent 20 coating each ofthe plurality of microparticles 16 to achieve adhesion under subsequentheating. In some embodiments, the dry dusting process is used when thesurface of base layer 12 comprises a reflective substrate comprising anadhesive layer that will dry, creating an adhesion and surfacemorphology. In some embodiments, the reflective substrate is glue orpaint, to name a couple of non-limiting examples. It is envisioned thatany suitable reflective substrate may be employed in the dry dustingprocess utilized in accordance with the embodiments of the presentdisclosure. In some embodiments, when base layer 12 comprising anadhesive layer is utilized in the dry dusting process, use of bindingagent 20 is optional.

In some embodiments, wet printing and fusing is utilized in lieu of theEMB process and dry dusting process. In some embodiments, the wetprinting and fusing process uses a printer to print a liquid suspensionof the plurality of microparticles 16 directly on the base layer 12. Insome embodiments, the wet printing and fusing process requiressubsequent heating through infrared heating or a hot roller process tocure the binding agent 20 coating each of the plurality ofmicroparticles 16 to achieve adhesion.

In one embodiment, the binding agent 20 layer shown in FIG. 1 includes acharacteristic thickness 23 between about 1 and about 50 microns in oneembodiment. It will be appreciated that the characteristic thickness 23may be greater than approximately 50 μm in other embodiments. It willfurther be appreciated that when the characteristic thickness 22 islarger than the characteristic dimension 18 of the plurality ofmicroparticles 16, a smooth surface morphology is created. Additionally,a rough surface morphology is created when the characteristic thickness23 is less than the characteristic dimension 18 of the plurality ofmicroparticles 16.

As shown in FIGS. 3 and 4, a computational study was performed to studythe cooling power of a composite material 10 in one embodiment composedof an aluminum-silver-silica (SiO₂) combination. The computational studyof composite material 10 (shown by line 24), yielded a cooling potentialof approximately 113 W/m² at an ambient temperature of approximately300° K when exposed to direct sunlight. The computational study alsoanalyzed an aluminum-silica combination, shown by line 26, and yielded acooling potential of approximately 82 W/m² at an ambient temperature ofapproximately 300° K when exposed to direct sunlight. For the idealcase, the computational study assumed a fictional ideal material that ispurely reflective in all bands other than 8-13 microns. In the 8-13micron band, the ideal material is purely emitting (i.e., emissivity=1).As shown, the aluminum-silver-silica combination outperforms the idealcase, shown by line 28, above an ambient temperature of approximately310° K due to a broader emission spectrum above approximately 13 μmthereby accessing narrower atmospheric window bands in the 20 to 25micron range.

As shown in FIG. 4, the computational study of thealuminum-silver-silica combination, shown by line 30, yields a coolingpotential exceeding approximately 250 W/m² when exposed to the nighttimesky and outperforms the ideal case, line 32, above an ambient nighttimetemperature of 255° K. It will therefore be appreciated that thecomposite material 10 includes at least one thermally-emissive layer 14bonded to a base layer 12, via a binding agent 20, where the compositematerial 10 produces a positive cooling potential in both daytime andnighttime ambient temperatures.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly certain embodiments have been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

What is claimed is:
 1. A method of providing a composite material for passive radiative cooling to a surface, said method comprising: obtaining access to an object to be cooled through passive radiative cooling, said object having a surface; applying to said surface a liquid suspension of microparticles in a liquid binding agent; and curing said binding agent so as to form a thermally-emissive layer on said surface, whereby said thermally-emissive layer is affixed to said surface via said binding agent.
 2. The method of claim 1, wherein said object comprises a base layer to which said liquid suspension is applied, said base layer comprising a reflective substrate.
 3. The method of claim 2, wherein said reflective substrate is composed of at least one of aluminum, silver, glass, polyurethane, nylon, and polyethylene fibers.
 4. The method of claim 2, wherein said reflective substrate comprises paint.
 5. The method of claim 1, wherein said binding agent is composed of a polymer material.
 6. The method of claim 1, wherein said binding agent is transparent.
 7. The method of claim 1, wherein said binding agent includes a characteristic thickness less than or equal to approximately 50 μm.
 8. The method of claim 1, wherein said at least one emissive layer is composed of silica material.
 9. The method of claim 1, wherein said at least one emissive layer comprises a plurality of microparticles.
 10. The method of claim 9, wherein each of said plurality of microparticles is composed of silica material.
 11. The method of claim 9, wherein each of said plurality of microparticles includes a characteristic dimension between about 5 to about 50 μm.
 12. The method of claim 9, wherein each of said plurality of microparticles includes a characteristic dimension less than or equal to 30 μm.
 13. The method of claim 1, wherein said liquid suspension is applied in the form of a spray.
 14. A method of providing a composite material for passive radiative cooling to a surface, said method comprising: obtaining access to an object to be cooled through passive radiative cooling, said object having a base layer; applying to said base layer a liquid suspension of microparticles in a liquid binding agent to said base layer, and curing said binding agent so as to form at least one thermally-emissive layer located adjacent to a surface of the base layer, wherein the surface of the base layer comprises a reflective substrate comprising an adhesive layer, and wherein the at least one emissive layer is affixed to the base layer via the adhesive layer of the base layer.
 15. The method of claim 14, wherein said reflective substrate comprises paint.
 16. The method of claim 14, wherein said reflective substrate comprises glue.
 17. The method of claim 14, wherein said at least one emissive layer comprises a silica material.
 18. The method of claim 14, wherein said at least one emissive layer comprises a plurality of microparticles.
 19. The method of claim 18, wherein each of said plurality of microparticles includes a characteristic dimension between about 5 to about 50 μm.
 20. The method of claim 18, wherein each of said plurality of microparticles includes a characteristic dimension less than or equal to 30 μm.
 21. The method of claim 14, wherein said liquid suspension is applied in the form of a spray. 